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CONFORMATIONAL ANALYSIS - University of Calgary CHEM 453

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Conformational Analysis An important aspect of organic compounds is that the compound is not static, but rather has conformational freedom by rotating, stretching and bending about bonds Each diffe... rent arrangement in space of the atoms is called a “Conformer” (a less used term is a “Rotamer” if change is caused by a bond rotation) Different conformers can have vastly different energies and the relative proportion of each conformer is related to the energy difference between them CH3 H3C H3C CH3 CH3 H H CH3 H H H H CH3 CH3 H H To observe the spatial arrangement of atoms in space, often draw conformers in a Newman projection Lower Energy 196$ Conformational Analysis Conformers will be of different energy due to strain Sources of strain are generally categorized in one of three types: 1) Torsional strain E n e r g y torsional angle -60˚ 0˚ 60˚ 120˚ 180˚ 240˚ Torsional strain is due to interactions as groups change relative position with a change in torsional bond angle Consider rotation of ethane H H H H H H H H H H H H Staggered conformation Eclipsed conformation Eclipsed is higher in energy than staggered due to increased torsional strain Further rotation will convert eclipsed conformation back to staggered H H H H H H ~3 kcal/mol 197$ Conformational Analysis 2) van der Waals strain Another source of strain is when groups are placed in positions closer than the sum of their van der Waals radii Consider rotation of butane E n e r g y torsional angle -60˚ 0˚ 60˚ 120˚ 180˚ 240˚ CH3 H H CH3 H H anti H H CH3 CH3 H H H H CH3 CH3 H H H H3C H CH3 H H gauche totally eclipsed eclipsed “gauche” conformation is higher in energy than anti (both are “staggered” conformations) “totally eclipsed” conformation (which has largest groups eclipsing each other) is higher in energy than other eclipsed conformations 198$ Conformational Analysis The difference in energy thus affects the amount of each conformer pr [Show More]

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